The present invention relates to a sheet which is proof against forgery by copying on a copying machine, hereinafter simply a copy-proof sheet, and more particularly relates to improvement in reproductivity of invisible latent marks incorporated in a copy-proof sheet on an obtained copy.
A copy-proof sheet is already known in the art in which one of a background and one or more latent marks such as "VOID" is formed of multi-line images and the other is formed of fine mesh images so that, when processed through a copying machine, only the multi-line images are reproduced on an obtained copy for providing visual recognition of the presence of the latent mark via difference in color tone.
In the case of the copy-proof technique, it is a basic requirement that a latent mark should never be seen on an original document but should appear distinctly from other sections on an obtained copy of the document in order to clearly indicate that the copy is actually a copy and not an original.
When the direction of lines in the multi-line images coincides with, i.e. is parallel to, the scanning direction of a copying machine, the multi-line images are clearly reproduced on an obtained copy. On the other hand, no good reproduction can be expected when the direction of the lines does not coincide with the scanning direction. When a copy-proof sheet is placed on a stack station of a copying machine, the direction of lines in its multi-line images does not always coincide with the scanning direction of the machine. In the use of such a sheet, the multi-line images are sometimes not clearly reproduced in deep color on an obtained copy due to such inconsistency in direction and, as a consequence, are not well distinguished from the fine mesh images. In other words, a latent mark and a background cannot easily be visually distinguished on an obtained copy.
In an attempt to avoid this inconvenience, it has already been proposed to orient lines in the multi-line images with an inclination of 45.degree. with respect to the longitudinal axis of a copy-proof sheet so that the multi-line images should be clearly reproduced on an obtained copy regardless of the position of the sheet placed on the stack station of a copying machine. In this case, however, the multi-line images may not be reproduced on the obtained copy as in the case of fine mesh images depending on the type of copying machine and/or the manner of color tone adjustment. Conversely, the multi-line images and the fine mesh images are both reproduced on the obtained copy when copied at a high production color tone.
It has also been proposed in the prior art to use a copy-proof sheet which includes latent marks formed of multi-line images of longitudinal lines only and latent marks formed of multi-line images of lateral lines only. Then, regardless of the position of the sheet placed on the stack station of a copying machine, one of the two groups of multi-line images coincides with the scanning direction of the machine and, as a consequence, at least the latent mark formed of that group of multi-line images will be distinctly reproduced on the obtained copy. This dual latent mark system is disclosed, for example, in U.S. Pat. No. 4,944,533 issued on July 31, 1990.
Due to indispensable inclusion of latent marks of two different groups, this system is quite unsuited for documents of relatively small size such as personal checks or traveler's checks. In addition, multi-line images of one group are not reproduced on the copy obtained. Even in the case of the multi-line images of the reproducible group, the latent marks reproduced may be camouflaged by other characters and/or patterns printed on an obtained copy depending on their locations.